Open WithCompletionHandler: 完成后如何返回结果

Question

想查询Coredata数据库中的一张照片

这是我的代码

这是 NSObjectSubclass 类别

//Photo+creak.h

#import "Photo+creat.h"

@implementation Photo (creat)

+(Photo *)creatPhotoByString:(NSString *)photoName inManagedObjectContext:(NSManagedObjectContext *)context{
    Photo *picture = nil;

    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Photo"];
    request.predicate = [NSPredicate predicateWithFormat:@"name = %@", photoName];

    NSArray *matches = [context executeFetchRequest:request error:nil];
    if (!matches || [matches count]>1) {
        //error
    } else if ([matches count] == 0) {
        picture = [NSEntityDescription insertNewObjectForEntityForName:@"Photo" inManagedObjectContext:context];
        picture.name = photoName;
    } else {
        picture = [matches lastObject];
    }
    return picture;
}

+ (BOOL)isPhoto:(NSString *)photoName here:(NSManagedObjectContext *)context{
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Photo"];
    request.predicate = [NSPredicate predicateWithFormat:@"name = %@", photoName];
    NSArray *matches = [context executeFetchRequest:request error:nil];
    switch ([matches count]) {
        case 1:
            return YES;
            break;
        default:
            return NO;
            break;
    }
}
@end

视图控制器内的代码

//View Controller
- (IBAction)insertData:(UIButton *)sender {
    NSURL *url = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
    url = [url URLByAppendingPathComponent:@"test"];
    UIManagedDocument *defaultDocument = [[UIManagedDocument alloc] initWithFileURL:url];
    if (![[NSFileManager defaultManager] fileExistsAtPath:[url path]]) {
        [defaultDocument saveToURL:defaultDocument.fileURL forSaveOperation:UIDocumentSaveForCreating completionHandler:NULL];
    }
    [defaultDocument openWithCompletionHandler:^(BOOL success) {
        [Photo creatPhotoByString:@"test" inManagedObjectContext:defaultDocument.managedObjectContext];
        [defaultDocument saveToURL:defaultDocument.fileURL forSaveOperation:UIDocumentSaveForOverwriting completionHandler:NULL];
    }];
    [sender setTitle:@"Okay" forState:UIControlStateNormal];
    [sender setEnabled:NO];
}

- (IBAction)queryFromDatabase:(UIButton *)sender {
    NSURL *url = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
    url = [url URLByAppendingPathComponent:@"test"];
    UIManagedDocument *defaultDocument = [[UIManagedDocument alloc] initWithFileURL:url];
    BOOL isItWorking = [checkPhoto isPhoto:@"test" inManagedDocument:defaultDocument];
    if (isItWorking) {
        [sender setTitle:@"Okay" forState:UIControlStateNormal];
    } else {
        [sender setTitle:@"NO" forState:UIControlStateNormal];
    }
}

连接它们的 NSObject 类。

 //  checkPhoto.m
#import "checkPhoto.h"
@implementation checkPhoto
+ (BOOL)isPhoto:(NSString *)photoToCheck inManagedDocument:(UIManagedDocument *)document{
    __block BOOL isPhotoHere = NO;
    if (document.documentState == UIDocumentStateClosed) {
        [document openWithCompletionHandler:^(BOOL success) {
            isPhotoHere = [Photo isPhoto:photoToCheck here:document.managedObjectContext];
        }];
    }
    return isPhotoHere;
}
@end

核心数据只有一个名为“照片”的实体，它只有一个属性“名称”。 问题是返回总是在块完成之前执行并且总是返回 NO。 测试代码here

或者我应该在查询时执行 openWithCompletionHandler 以外的其他操作吗？

Answer 1

您需要重新设计您的方法以异步工作，例如-openWithCompletionHandler:。它需要采用一个块，该块在已知答案时被调用，并接收答案，真或假，作为参数。

然后，调用者应该传入一个块，该块执行已知答案后应该发生的任何事情。

或者，或者，您应该延迟整个逻辑块，它关心照片是否在数据库中。应该在打开完成后完成。

您必须为更具体的建议显示更多代码。

---

因此，您可以将 isPhoto... 方法修改为：

+ (BOOL)checkIfPhoto:(NSString *)photoToCheck isInManagedDocument:(UIManagedDocument *)document handler:(void (^)(BOOL isHere))handler {
    if (document.documentState == UIDocumentStateClosed) {
        [document openWithCompletionHandler:^(BOOL success) {
            handler([Photo isPhoto:photoToCheck here:document.managedObjectContext]);
        }];
    }
    else
        handler(NO);
}

然后你可以重做这个：

- (IBAction)queryFromDatabase:(UIButton *)sender {
    NSURL *url = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
    url = [url URLByAppendingPathComponent:@"test"];
    UIManagedDocument *defaultDocument = [[UIManagedDocument alloc] initWithFileURL:url];
    [checkPhoto checkIfPhoto:@"test" isInManagedDocument:defaultDocument handler:^(BOOL isHere){
        if (isHere) {
            [sender setTitle:@"Okay" forState:UIControlStateNormal];
        } else {
            [sender setTitle:@"NO" forState:UIControlStateNormal];
        }
    }];
}

Answer 2

试试看

+(BOOL)isPhoto:(Photo *)photo inDataBase:(UIManagedDocument *)defaultDocument{
 __block BOOL isPhotoThere = NO;
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
[defaultDocument openWithCompletionHandler:^(BOOL success) {
    [defaultDocument.managedObjectContext performBlock:^{
        isPhotoThere = [Photo checkPhoto:photo];
        dispatch_semaphore_signal(sema);
    }];
 }];
 dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
 dispatch_release(sema);
 return isPhotoThere;
}